This invention relates to a device or a gun for painting by electrostatic powder spraying.
Conventionally known are various types of devices for painting by electrostatic powder spraying, wherein, in general, a corona discharge is utilized for electrifying powder particles so as to electrostatically adhere said powder particles to the surface to be coated. However, according to tests conducted by the inventors of the present invention, only a small percentage of the corona ions is utilized for electrifying the powder particles, and therefore, the electrifying efficiency is low. When the amount of the charge on the powder particles is small, the deposition efficiency is low. More specifically, the deposition efficiency is only 70% for a large plane surface to be coated. If the amount of the charge on the powder particles is increased so as to increase the deposition efficiency, a considerably higher voltage is required to be supplied to an electrode for creating the corona discharge, because the amount of the charge is proportional to the supplied voltage. Accordingly, in conventionally known devices for painting by electrostatic powder spraying, a high voltage, i.e., between 60 KV and 90 KV, is supplied to an electrode for creating the corona discharge. Under such a high voltage, a very large corona current flows.
However, such a high voltage creates problems concerning safety, i.e., the problem of danger when a person touches the electrode and the problem of dust explosion. In addition, when a high voltage is supplied to an electrode so as to discharge the corona, an amount of ions which exceeds the electric breakdown limit may be stored in the deposited powder layer on the surface to be coated, because the corona current is large. As a result, back ionization occurs in the deposited powder layer and crator-like defects are frequently formed in the coated surface. Also, the deposition efficiency is extremely low when the thickness of the powder layer exceeds a certain value.